1. Field of the Invention
This invention relates to the thermal toughening of glass by contacting te glass with a fluidised bed of particulate material whose teperature, relative to the temperature of the glass, is such that there is heat-exchange between the glass and the particulate material.
More particularly the invention relates to the thermal toughening of glass articles, for example flat or curved glass sheets, by immersing the articles in a fluidised bed of particulate material.
2. Description of the Prior Art
In U.S. Pat. No. 4,113,458 there are described a method and apparatus for thermally treating glass articles by quenching the articles, in turn, in a gas-fluidised bed of particulate material which is placed in a quiescent uniformly expanded state of particulate fluidisation by control of the distribution of fluidising gas in the particulate material at a gas flow velocity through the particulate material between that velocity corresponding to incipient fluidisation and that velocity corresponding to maximum expansion of the particulate material.
The method of this application is particularly efficacious for the thermal toughening of flat or curved sheets of glass which are at a temperature above the strain point of the glass, and are immersed in the fluidised bed where heat exchange with the fluidised particulate material engenders toughening stresses in the glass. This method has been employed for the thermal toughening of curved sheets of glass which are to be used as one component of a laminated glass automobile windscreen.
The quiescent surface of the bed which the hot glass sheet enters ensures that the lower edge of the sheet is uniformly chilled as the lower edge of the sheet enters the fluidised bed.
As the hot glass sheet enters the particulate material there is agitation of the particulate material in the vicinity of the glass surfaces which ensures that there is adequate heat transfer away from the glass surfaces into the bulk of the fluidised bed, dependent on the rate of movement of particles which have become heated in proximity to the glass surfaces away from the vicinity of the glass surfaces with the concurrent supply of cooler particles into the vicinity of the glass surfaces from the bulk of the fluidised bed.
It has now been discovered that materials such as the porous .gamma.-aluminas and the porous aluminosilicates as disclosed in U.S. Pat. No. 4,113,458 are particularly effective for the thermal toughening of glass because such materials have gas-evolution properties when heated. These materials hve water adsorbed in their pores and the gas driven off is water vapour when the particulate material is heated in the vicinity of the glass surfaces.
The release of gas from such particulate materials when heated in the vicinity of the glass surfaces is now considered to be a basic fctor in producing the rapid agitation of the particulate material which occurs at the glass surfaces when the glass is toughened by immersion in such materials. The rapid agitation ensures that there is a sufficient amount of heat transfer from the glass surfaces into the bulk of the fluidised bed to give the higher values of central tensile stress which it was found possible to induce in glass sheets.
The selection of a material having gas-generating properties is not however sufficient in itself for the attainment of higher toughening stresses and other factors are involved. It has now been found that in order to obtain full benefit from the use of a material having gas-generating properties, which material is maintained in a quiescent uniformly expanded state of particulate fluidisation, it is important to select the mean particle size, particle size distribution, and the flowability of the material, as defined below.
The generation of gas from the particulate material can then induce a sufficient rapidity of movement of the particulate material in the vicinity of the glass surfaces to maximise heat transfer by movement of heat particles away from the glass surfaces while cooler particles are supplied continuously from the bulk of the fluidised material into the vicinity of the glass surfaces.
The "flowability" of a particulate material can be expressed as a number which is the sum of four point scores which are awarded to the material by assessment of four characteristics of the particulate material, and the term "flowability" when used herein has that meaning.
These four characteristics of a flowable particulate material and the manner of awarding point scores are described in the article "Evaluating Flow Properties of Solids" by Ralph L. Carr Jr., Chemical Engineering Volume 72, Number 2, Jan. 18, 1965 and are as follows:
1. Compressibility=100(P-A)/P% where P=packed bulk density and A=aerated bulk density
2. Angle of Repose: this is the angle in degrees between the horizontal and the slope of a heap of the particulate material dropped from a point above the horizontal until a constant angle is measured.
3. Angle of Spatula: a spatula is inserted horizontally into the bottom of a mass of the dry particulate material and is lifted straight up and out of the material. An average value of the angle in degrees to the horizontal of the side of the heap of material on spatula is the Angle of Spatula.
4. Particle Size Distribution (called Uniformity Co-efficient in the above mentioned article): this is described in the above mentioned article as the numerical value arrived at by dividing the width of sieve opening (i.e. particle size) which will pass 60% of the particulate material by the width of sieve opening which will just pass 10% of the particulate material.
All the values of particle size distribution referred to herein were measured in known manner by an equivalent method using a Coulter counter to determine the particle diameters appropriate to cumulative weight percentages of 40% and 90% corresponding to widths of sieve openings which will pass 60% and will just pass 10% of the particulate material.
The numerical values of Compressibility, Angle of Repose, and Angle of Spatula were measured using a Hosokawa Powder Tester manufactured by the Hosokawa Micrometrics Laboratory of The Hosokawa Iron Works, Osaka, Japan, which Powder Tester is specifically designed for use in the determination of the "flowability" of powders as defined above.